Visual cryptography is a secret sharing method that uses human eyes to decrypt the secret. Most visual cryptographic methods utilize the technique of pixel expansion, which causes the size of the shares to be much larger than that of the secret image. This situation is more serious for grey-level and chromatic images. In this paper, we propose a multi-pixel encoding method for grey-level and chromatic images without pixel expansion. We simultaneously encrypt r successive white or black pixels each time. The probability of these r pixels being coloured black depends on the ratio of blacks in the basis matrices. Afterward, we incorporate the techniques of colour decomposition and halftoning into the proposed scheme to handle grey-level and chromatic images. The experimental results show that the shares are not only the same size as the secret image, but also attain the requirement of security. The stacked images have good visual effect as well. Besides, our method can be easily extended to general access structure.
Journal of Research and Practice in Information Technology 37(2), pp.179-191